Fix FastLazy race condition and waiting thread hanging (#6336)

src/benchmark/Akka.Benchmarks/Utils/FastLazyBenchmarks.cs

@@ -19,16 +19,13 @@ public class FastLazyBenchmarks
         private Lazy<int> lazySafe;
         private Lazy<int> lazyUnsafe;
         private FastLazy<int> fastLazy;
-        private FastLazy<int, int> fastLazyWithInit;
-
 
         [GlobalSetup]
         public void Setup()
         {
             lazySafe = new Lazy<int>(() => 100, LazyThreadSafetyMode.ExecutionAndPublication);
             lazyUnsafe = new Lazy<int>(() => 100, LazyThreadSafetyMode.None);
             fastLazy = new FastLazy<int>(() => 100);
-            fastLazyWithInit = new FastLazy<int, int>(state => state + 100, 1000);
         }
 
         [Benchmark(Baseline = true)]
@@ -48,11 +45,5 @@ public int FastLazy_get_value()
         {
             return fastLazy.Value;
         }
-
-        [Benchmark]
-        public int FastLazy_stateful_get_value()
-        {
-            return fastLazyWithInit.Value;
-        }
     }
 }

src/core/Akka.API.Tests/verify/CoreAPISpec.ApproveCore.DotNet.verified.txt

@@ -4996,14 +4996,8 @@ namespace Akka.Util
     public sealed class FastLazy<T>
     {
         public FastLazy(System.Func<T> producer) { }
-        public bool IsValueCreated { get; }
-        public T Value { get; }
-    }
-    public sealed class FastLazy<S, T>
-    {
-        public FastLazy(System.Func<S, T> producer, S state) { }
-        public bool IsValueCreated { get; }
         public T Value { get; }
+        public bool IsValueCreated() { }
     }
     public interface ISurrogate
     {

src/core/Akka.Tests/Util/FastLazySpecs.cs

@@ -0,0 +1,118 @@
+//-----------------------------------------------------------------------
+// <copyright file="FastLazySpecs.cs" company="Akka.NET Project">
+//     Copyright (C) 2009-2023 Lightbend Inc. <http://www.lightbend.com>
+//     Copyright (C) 2013-2023 .NET Foundation <https://github.com/akkadotnet/akka.net>
+// </copyright>
+//-----------------------------------------------------------------------
+
+using System;
+using System.Collections.Concurrent;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using Akka.Util;
+using Xunit;
+
+namespace Akka.Tests.Util;
+
+public class FastLazySpecs
+{
+    [Fact]
+    public void FastLazy_should_indicate_no_value_has_been_produced()
+    {
+        var fal = new FastLazy<int>(() => 2);
+        Assert.False(fal.IsValueCreated());
+    }
+
+    [Fact]
+    public void FastLazy_should_produce_value()
+    {
+        var fal = new FastLazy<int>(() => 2);
+        var value = fal.Value;
+        Assert.Equal(2, value);
+        Assert.True(fal.IsValueCreated());
+    }
+
+    [Fact]
+    public void FastLazy_must_be_threadsafe()
+    {
+        for (var c = 0; c < 100000; c++) // try this 100000 times
+        {
+            var values = new ConcurrentBag<int>();
+            var fal = new FastLazy<int>(() => new Random().Next(1, Int32.MaxValue));
+            var result = Parallel.For(0, 1000, i => values.Add(fal.Value)); // 1000 concurrent operations
+            SpinWait.SpinUntil(() => result.IsCompleted);
+            var value = values.First();
+            Assert.NotEqual(0, value);
+            Assert.True(values.All(x => x.Equals(value)));
+        }
+    }
+
+    [Fact]
+    public void FastLazy_only_single_value_creation_attempt()
+    {
+        int attempts = 0;
+        Func<int> slowValueFactory = () =>
+        {
+            Interlocked.Increment(ref attempts);
+            Thread.Sleep(100);
+            return new Random().Next(1, Int32.MaxValue);
+        };
+
+        var values = new ConcurrentBag<int>();
+        var fal = new FastLazy<int>(slowValueFactory);
+        var result = Parallel.For(0, 1000, i => values.Add(fal.Value)); // 1000 concurrent operations
+        SpinWait.SpinUntil(() => result.IsCompleted);
+        var value = values.First();
+        Assert.NotEqual(0, value);
+        Assert.True(values.All(x => x.Equals(value)));
+        Assert.Equal(1000, values.Count);
+        Assert.Equal(1, attempts);
+    }
+
+    [Fact]
+    public void FastLazy_must_be_threadsafe_AnyRef()
+    {
+        for (var c = 0; c < 100000; c++) // try this 100000 times
+        {
+            var values = new ConcurrentBag<string>();
+            var fal = new FastLazy<string>(() => Guid.NewGuid().ToString());
+            var result = Parallel.For(0, 1000, i => values.Add(fal.Value)); // 1000 concurrent operations
+            SpinWait.SpinUntil(() => result.IsCompleted);
+            var value = values.First();
+            Assert.NotNull(value);
+            Assert.True(values.All(x => x.Equals(value)));
+        }
+    }
+
+    [Fact]
+    public void FastLazy_only_single_value_creation_attempt_AnyRef()
+    {
+        int attempts = 0;
+        Func<string> slowValueFactory = () =>
+        {
+            Interlocked.Increment(ref attempts);
+            Thread.Sleep(100);
+            return Guid.NewGuid().ToString();
+        };
+
+        var values = new ConcurrentBag<string>();
+        var fal = new FastLazy<string>(slowValueFactory);
+        var result = Parallel.For(0, 1000, i => values.Add(fal.Value)); // 1000 concurrent operations
+        SpinWait.SpinUntil(() => result.IsCompleted);
+        var value = values.First();
+        Assert.NotNull(value);
+        Assert.True(values.All(x => x.Equals(value)));
+        Assert.Equal(1000, values.Count);
+        Assert.Equal(1, attempts);
+    }
+
+    [Fact]
+    public void FastLazy_AllThreads_ShouldThrowException_WhenFactoryThrowsException()
+    {
+        var lazy = new FastLazy<string>(() => throw new Exception("Factory exception"));
+        var result = Parallel.For(0, 10, i => { Assert.Throws<Exception>(() => _ = lazy.Value); });
+
+        Assert.True(result.IsCompleted);
+    }
+}

src/core/Akka/Util/FastLazy.cs

@@ -15,144 +15,55 @@ namespace Akka.Util
     /// A fast, atomic lazy that only allows a single publish operation to happen,
     /// but allows executions to occur concurrently.
     /// 
-    /// Does not cache exceptions. Designed for use with <typeparamref name="T"/> types that are <see cref="IDisposable"/>
-    /// or are otherwise considered to be expensive to allocate. 
-    /// 
-    /// Read the full explanation here: https://github.com/Aaronontheweb/FastAtomicLazy#rationale
+    /// Does not cache exceptions. Designed for use with <typeparam name="T"/> types that are <see cref="IDisposable"/>
+    /// or are otherwise considered to be expensive to allocate. Read the full explanation here: https://github.com/Aaronontheweb/FastAtomicLazy#rationale
     /// </summary>
-    /// <typeparam name="T">TBD</typeparam>
     public sealed class FastLazy<T>
     {
-        private readonly Func<T> _producer;
-        private byte _created = 0;
-        private byte _creating = 0;
+        private Func<T> _producer;
+        private int _status = 0;
+        private Exception _exception;
         private T _createdValue;
 
-        /// <summary>
-        /// Initializes a new instance of the <see cref="FastLazy{T}"/> class.
-        /// </summary>
-        /// <exception cref="ArgumentNullException">
-        /// This exception is thrown if the given <paramref name="producer"/> is undefined.
-        /// </exception>
         public FastLazy(Func<T> producer)
         {
-            if (producer == null) throw new ArgumentNullException(nameof(producer), "Producer cannot be null");
-            _producer = producer;
+            _producer = producer ?? throw new ArgumentNullException(nameof(producer));
         }
 
-        /// <summary>
-        /// TBD
-        /// </summary>
-        /// <returns>TBD</returns>
-        public bool IsValueCreated => IsValueCreatedInternal();
-
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        private bool IsValueCreatedInternal()
-        {
-            return Volatile.Read(ref _created) == 1;
-        }
-
+        public bool IsValueCreated() => Volatile.Read(ref _status) == 2;
+
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        private bool IsValueCreationInProgress()
-        {
-            return Volatile.Read(ref _creating) == 1;
-        }
-
-        /// <summary>
-        /// TBD
-        /// </summary>
+        private bool IsExceptionThrown() => Volatile.Read(ref _exception) != null;
+
         public T Value
         {
             get
             {
-                if (IsValueCreatedInternal())
+                if (IsValueCreated())
                     return _createdValue;
-                if (!IsValueCreationInProgress())
-                {
-                    Volatile.Write(ref _creating, 1);
-                    _createdValue = _producer();
-                    Volatile.Write(ref _created, 1);
-                }
-                else
+
+                if (Interlocked.CompareExchange(ref _status, 1, 0) == 0)
                 {
-                    SpinWait.SpinUntil(IsValueCreatedInternal);
-                }
-                return _createdValue;
-            }
-        }
-    }
-
-
-    /// <summary>
-    /// A fast, atomic lazy that only allows a single publish operation to happen,
-    /// but allows executions to occur concurrently.
-    /// 
-    /// Does not cache exceptions. Designed for use with <typeparamref name="T"/> types that are <see cref="IDisposable"/>
-    /// or are otherwise considered to be expensive to allocate. 
-    /// 
-    /// Read the full explanation here: https://github.com/Aaronontheweb/FastAtomicLazy#rationale
-    /// </summary>
-    /// <typeparam name="S">State type</typeparam>
-    /// <typeparam name="T">Value type</typeparam>
-    public sealed class FastLazy<S, T>
-    {
-        private readonly Func<S, T> _producer;
-        private byte _created = 0;
-        private byte _creating = 0;
-        private T _createdValue;
-        private S _state;
-
-        /// <summary>
-        /// Initializes a new instance of the <see cref="FastLazy{T}"/> class.
-        /// </summary>
-        /// <exception cref="ArgumentNullException">
-        /// This exception is thrown if the given <paramref name="producer"/> or <paramref name="state"/> is undefined.
-        /// </exception>
-        public FastLazy(Func<S, T> producer, S state)
-        {
-            if(producer == null) throw new ArgumentNullException(nameof(producer), "Producer cannot be null");
-            if(state == null) throw new ArgumentNullException(nameof(state), "State cannot be null");
-            _producer = producer;
-            _state = state;
-        }
-
-        /// <summary>
-        /// TBD
-        /// </summary>
-        /// <returns>TBD</returns>
-        public bool IsValueCreated => IsValueCreatedInternal();
-
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        private bool IsValueCreatedInternal()
-        {
-            return Volatile.Read(ref _created) == 1;
-        }
-
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        private bool IsValueCreationInProgress()
-        {
-            return Volatile.Read(ref _creating) == 1;
-        }
+                    try
+                    {
+                        _createdValue = _producer();
+                    }
+                    catch (Exception e)
+                    {
+                        Volatile.Write(ref _exception, e);
+                        throw;
+                    }
 
-        /// <summary>
-        /// TBD
-        /// </summary>
-        public T Value
-        {
-            get
-            {
-                if (IsValueCreatedInternal())
-                    return _createdValue;
-                if (!IsValueCreationInProgress())
-                {
-                    Volatile.Write(ref _creating, 1);
-                    _createdValue = _producer(_state);
-                    Volatile.Write(ref _created, 1);
-                    _state = default(S); // for reference types to make it suitable for gc
+                    Volatile.Write(ref _status, 2);
+                    _producer = null; // release for GC
                 }
                 else
                 {
-                    SpinWait.SpinUntil(IsValueCreatedInternal);
+                    SpinWait.SpinUntil(() => IsValueCreated() || IsExceptionThrown());
+                    var e = Volatile.Read(ref _exception);
+                    if (e != null)
+                        throw e;
                 }
                 return _createdValue;
             }